Recently, a semiconductor device using a GaN-based material such as a blue light emitting diode (LED), a white LED, a blue-violet semiconductor laser (LD), or a high frequency device has been attracted.
The GaN-based semiconductor device is realized by heteroepitaxial growth of a GaN-based thin film deposited on a substrate formed of silicon carbide (SiC) or sapphire by an organic metal vapor phase epitaxy (MOVPE) method or a molecular beam epitaxy (MBE) method. In this case, since the thermal expansion coefficients and the lattice constants of the GaN-based thin film and the substrate are significantly different from each other, dislocation (that is, a defect) having high density of 109 cm−2 or more occurs in the GaN-based thin film.
Meanwhile, in order to fabricate the white LED, the blue-violet semiconductor laser (LD), and the high frequency device, the development of a high-quality GaN substrate having dislocation density of 104 cm−2 or less is required and various technologies have been developed in various research institutions (for example, Non-Patent Documents 1 and 2).
Non-Patent Document 1 is a research report on GaN crystal growth by a molten liquid growth method, in which it was confirmed by experiments that, when GaN is congruently melted without decomposition at a nitrogen pressure of 6 GPa (about 60,000 atm) or more and a high temperature of 2220° C. or above and the molten liquid is cooled, the molten liquid is reversibly returned to a GaN crystal phase.
Non-Patent Document 2 is a research report on GaN crystal growth by a flux method, in which it was confirmed by experiments that high-quality GaN crystal can be grown under growth conditions of a temperature of 800° C. and a nitrogen pressure of 5 MPa (about 50 atm).
A method for activating an impurity ion implanted layer for semiconductor such as SiC or GaN is disclosed in Patent Document 1.
In the “method for activating the impurity ion implanted layer” of Patent Document 1, laser light whose photon energy is equal to or higher than the photon energy (transition energy between band gaps) of a semiconductor material is irradiated to the semiconductor material, to which predetermined impurity elements are doped by ion implantation, in a state in which the semiconductor material is heated.
[Non-Patent Document 1]
Wataru UTUMI, et al., “A new method for congruent melting gallium nitride at a high pressure and growing single crystal”, Spring-8 user information, January, 2004
[Non-Patent Document 2]
Shoji SARAYAMA, et al., “High-quality crystal growth of gallium nitride by a flux method”, Ricoh Technical Report No. 30, December, 2004
[Patent Document 1]
Japanese Laid-open Patent Application Publication No. 2002-289550, “A method for activating an impurity ion implanted layer”
Since the melting point of GaN is approximately 2220° C. or more and the equilibrium pressure with nitrogen gas at this melting point reaches about 6 GPa (about 60,000 atm) or more, GaN decomposes to Ga metal and nitrogen gas at a high temperature under a low-pressure nitrogen atmosphere and thus single crystal growth means for obtaining single crystal by slowly cooling the molten liquid performed by silicon or the like cannot be applied.
Although the GaN crystal can be grown by the molten liquid growth method of the Non-Patent Document 1, there has been a problem that since the nitrogen pressure of 6 GPa (about 60,000 atm) or more and the high temperature of 2220° C. or more are required, an ultra-high temperature/ultra-high pressure durable apparatus is required.
In the flux method of Non-Patent Document 2, the equilibrium vapor pressure at a temperature of 600 to 800° C. can be reduced to several tens of atm, but a high-pressure device is required.
As in Patent Document 1, even when silicon carbide or gallium nitride is annealed by pulsed-laser light, melting cannot be performed at a vacuum atmosphere or a nitrogen atmosphere. Thus, it is difficult to accomplish high-quality crystal growth.